1. Field of the Invention
The present invention relates to a projector having a light source, an optical modulator for modulating light beam emitted from the light source according to image information and forming an optical image, and a projection lens, which has a plurality of lenses and a lens barrel for containing the plurality of lenses thereinside, for enlarging and projecting the optical image formed by the optical modulator.
2. Description of Related Art
Projectors have been and being popularly used for the purpose of presentation in conferences, academic meetings, exhibitions and so on and also for the purpose of enjoying movies or the like at home. Such projectors include a light source, an optical modulator for modulating light beam emitted from the light source according to image information and forming an optical image and a projection lens for enlarging and projecting the optical image formed by the optical modulator.
While it is ideal for such projectors to project an optical image in a direction perpendicular to the plane of projection such as a screen, they are often made to project an optical image obliquely onto the plane of projection from above or from below in actual applications. Then, there arises a problem of trapezoidal distortion that the projected image is distorted to show an upwardly or downwardly expanding trapezoidal contour. When a trapezoidal distortion occurs, the projected image goes out of focus and becomes blurred partially either in an upper area or in a lower area thereof because the image forming position of the projected image shows a positional discrepancy between the upper area and the lower area.
Projectors adapted to vertically shift the projection lens have been proposed to cope with this problem (see, for example, Japanese Patent Laid-Open Publication No. 2003-315648). Such projectors can dissolve the problem of projecting a partially burred image by vertically translating the projection lens and adjusting image forming position of the projected image relative to the plane of projection. As a result, it is possible to clearly project an optical image.
However, with the technique described in the above-cited patent document, the central axis of the light beam emitted from the optical modulator and entering the projection lens is moved away from the optical axis of the projection lens as the projection lens is translated vertically so that consequently the focal position of the projection lens can be moved out of the image forming region of the optical modulator. More specifically, the focal length of the projection lens that is employed for a projector tends to be decreased as the height of the projected image is increased. Thus, as the projection lens is translated vertically to increase the distance between the central axis of the light beam emitted from the optical modulator and the optical axis of the projection lens, it may be no longer possible to place the focal position of the projection lens in the image forming region of the optical modulator. Then, there arises a problem of degradation of the projected image due to the field curvature of the projection lens.
While it may be conceivable to use a projection lens that is free from field curvature, such a lens is costly and hence the cost of manufacturing a projector having such a projection lens will become prohibitive.